Gamma Doses - doe-std-1136-20040152

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

The data in Figure 6-3 were obtained with a tissue equivalent plastic scintillation detector and

demonstrate the spectral changes and the resultant exposure rates under typical protective clothing. It

can be seen from Figures 6-2 and 6-3 that significant fractions of the uranium beta radiation will

penetrate typical protective clothing worn in facilities which process uranium.

6.2.2 Gamma Doses

Gamma radiation from uranium is normally not the controlling challenge to radiation protection.

For example, the contact beta radiation field from depleted uranium is approximately 240 mrem/h, while

the contact gamma radiation field is less than 10 mrem/h. Although gamma radiation fields from uranium

are not usually the dominant concern, significant gamma fields can exist in areas where large quantities of

uranium are stored. Bremsstrahlung from the 2.29 MeV 234mPa beta can contribute up to 40% of the photon

dose from uranium metal. Neutron fields from enriched uranium fluoride compounds can also add to this

area of concern. Care should be taken that dose-equivalents from such fields are kept to levels that are

ALARA.

Although beta radiation fields from unshielded uranium tend to present the most intense radiation

problem, storage of large quantities of uranium can create widespread, low-level (<5 mrem/h) gamma

radiation fields. Such fields can create ALARA problems--particularly when significant numbers of

people must work in adjacent areas.

6.2.3 Neutron Dose Equivalents

In uranium processes that create fluoride compounds (UF4, UF6, etc.), the a-n reaction with this

light nuclide can result in neutron radiation fields, the intensity of which are a function of the compound,

mixing, storage configuration, and enrichment. As indicated in Section 2.0, low enriched UF6 (< 5%) in

large storage containers can result in neutron radiation in the 0.2 mrem/h range, while highly enriched (>

97%) UF6 can create fields in the 4 mrem/h range. At high enrichments, the neutron fields can be up to a

factor of 2 higher than the gamma fields and be the limiting source of whole body exposure. Neutron

radiation from uranium metals and low enriched compounds is considerably lower than the gamma

component and, consequently, is not limiting.

Neutron dose equivalent rates can be calculated accurately with computer codes, such as MCNP

(Breismeier 1986). The MCNP code has the advantage that it can calculate both neutron and photon doses

through shielding and in comple x arrays. The Monte Carlo codes can also calculate the effects of neutron

multiplication in systems containing large amounts of uranium. However, neutron dose equivalent rates

can also be calculated from simple empirical formulas. Unlike gamma doses, there is very little self-

shielding for neutrons in sub-kilogram masses of uranium.

Table 6-4 lists spontaneous fission yields for uranium isotopes that may be found in facilities

within the DOE complex. These data are taken from NUREG/CR-5550 (NRC 1991) and are believed to be

more current then the previously published PNL values (PNL 1988b). As a rule of thumb, nuclides with

even numbers of protons and neutrons have the highest spontaneous fission neutron emission rates. The

spontaneous fission rate for odd-even nuclides is about 1000 times less, and the rate for odd-odd

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